Isolation, purification and physico-chemical and functional characterization of the "middle": component(s) of the shark complement system, reacting between C1n and the two terminal components, t1 and t2. Elucidation of the relation between the first component of shark complement (Cln) and the C4 inactivator (C4I) from shark serum. Analysis of the C4I precursor system. Mechanism of C4 inactivation by C4I. Isolation and characterization of the mammalian serum factor(s) that inactivate C4I. Evaluation of the effect of the non-C-fixing natural and immune antibodies of the shark on a tumor transplant system in mice. Further studies regarding the new concept of "immune chemotaxis", initiated by immune reactions occurring on the surface of the responding cells (PMNs and macrophages).